The invention pertains to superconducting magnets and power supplies for superconducting devices such as superconducting magnets.
A superconducting device comprises a superconducting element (such as a superconducting ceramic coil) that is contained within a cryogenic chamber and that is driven by a power supply. The power supply is typically maintained at ambient temperature, e.g., 300.degree. K., while the cryogenic chamber (and the superconducting devices therein) is maintained at the temperature of liquid nitrogen (77.degree. K.) or below. Particularly, for ceramic superconducting elements, the chamber is maintained at so-called high temperature superconducting (HTS) ranges, to wit, 20.degree.-110.degree. K., while for metallic superconducting elements, the chamber is maintained at low temperature superconducting (LTS) temperature ranges, to wit, 4.degree.-20.degree. K.
Differences between the power supply temperature and cryogenic chamber temperature makes supplying power to the chamber difficult. Traditionally, power has been carried by conventional metallic wire leads (or joints) connecting the ambient temperature power supply to the cryogenic chamber. Use of such leads, however, makes it difficult (and expensive) to maintain a low temperature in the chamber. Particularly, if the leads have a thick cross section, they conduct a large amount of heat from the ambient environment into the chamber. If the cross section of the leads is reduced, the amount of heat transferred by conduction is lessened, although increased resistance in the metal results in heat input via resistive heating. One prior art solution for minimizing conductive and resistive heating is to cool the leads with cryogenic vapors, e.g., vapor from liquid helium; see Efferson, "Helium Vapor Cooled Current Leads," The Review of Scientific Instruments, Vol. 38, No. 12, December 1967.
The prior art suggests an additional approach for LTS applications. Rather than driving current through lossy conductive leads, it suggests using "flux pumps" to inductively pass power across the cryogenic barrier. In a nutshell, an AC-powered coil is placed on the outside of the chamber in order to induce alternating current flow in another coil on the inside of the chamber. A rectifier within the chamber converts the induced power to direct current (DC). See, TenKate, "Superconducting Rectifiers" (Thesis), Twente University, Apr. 6, 1984.
Thermally-activated superconductive switches are typically used to control the flow of current from the rectifier to the superconducting load. The switches typically constitute a material that is conductive at LTS temperatures, but nonconductive at higher temperatures. Those higher temperatures, which are only slightly above the LTS temperature, can be induced via small resistive elements disposed about the switches and activated via thin, low-current leads passing into the chamber. In LTS applications, e.g., superconducting magnets with metallic and intermetallic coils, these switches can be "thrown" with little heat input to the cryogenic chamber.
Flux pump technology has traditionally been thought to provide little value in HTS applications, e.g., superconducting magnets with ceramic coils. This is because thermally-activated superconductive switches operational in HTS temperature ranges are believed to require too much heat for transition between the open and closed states. Moreover, these switches, are not believed to change state rapidly enough to permit adequate control of superconducting ceramic coils and other HTS loads.
Where superconducting coils are employed as magnets, e.g., for medical magnetic resonance imaging, there is typically a need for constant magnetic field output. Metallic LTS superconducting coils are often used for these applications, since they are "persistent," i.e., they maintain constant current flows without additional applied power. Leads used to drive power into those coils can typically be disconnected to reduce heat loss, once coils have achieved the desired persistent states.
In contrast, magnets fashioned from superconducting ceramics or other HTS materials experience a phenomenon referred to as "flux creep." In these devices, a finite voltage drop across the device (e.g., NMR insert) necessitates permanent connection of the current leads and at least intermittent application of additional power.
In view of the foregoing, an object of this invention is to provide an improved power supply for superconducting devices, and, more particularly, a power supply that permits adequate powering and control of superconducting loads (such as coils) without adding excessive heat to the cryogenic chamber.
Another object to the invention is to provide a power supply for superconducting loads that can compensate for the "flux creep" traditionally experienced by ceramic superconducting loads and other HTS elements, without introducing a large amount of heat through permanently connected high current leads.
Still another object to the invention is to provide an improved power supply for metallic and intermetallic superconductors, or other LTS loads.
Yet further aspects of the invention are to provide improved superconducting magnets.
Still other objects of the invention are to provide such power supplies as can be readily constructed without great cost.